This invention relates to the field of air cleaners, and more particularly to such cleaners which include an outer, principal filter element enclosing an inner, "safety" filter element. In such arrangements the safety filter under ordinary conditions is functionally redundant, but it is provided to give the cleaner added reliability: in case of puncture or rupture of the principal filter, the safety filter prevents unfiltered air with its particulate contaminants from reaching the engine or other device which the cleaner is to protect until the principal filter is replaced. Some matter does, nevertheless, pass through the principal filter, particularly after an extended period of use, and is trapped by the safety filter. Principal filters are generally replaced according to some pragmatically determined schedule, or alternatively when a differential pressure indicator of some sort shows that the buildup of particulate matter in the cleaner as a whole has reached an unacceptable level. It would be possible to replace a safety filter each time a principal filter is replaced, but that expedient is not economically justifiable, and in fact reduces the overall system reliability by increasing the amount of time during which there is no protection at all at the system intake. An arbitrary schedule is usually resorted to which calls for the safety filter to be replaced, for example, at every third replacement of a principal filter. Here again, excessive caution generally causes replacement of the safety filter before this is functionally justified. No practical means has heretofore been available to determine when in fact the safety filter needs to be replaced: the known differential pressure indicators give the pressure drop across the entire air cleaner, but cannot distinguish as to how much of a pressure drop is due to a particular one of the two filters.